This experiment was conducted to evaluate the effects of a commercial dietary β-mannanase enzyme on growth performance, immunity, gut integrity and salinity tolerance of spinefoot rabbitfish (Siganus rivulatus) juveniles fed soybean-based diets. A basal diet (protein, 400 g/kg; gross energy, 20 MJ/kg) was supplemented with 0.0 (control), 0.2, 0.5 or 0.8 g β-mannanase/kg to formulate four experimental diets. Each treatment contained three net cages with 50 fish per net. The experiment lasted for 8 weeks. The results revealed that all β-mannanase levels have significantly improved the growth performance compared with the control. β-mannanase addition at 0.5 g/ kg diet showed the best results of the growth performance, survival rate and feed utilization efficiency. The results also indicated that β-mannanase supplementation improved protease, lysozyme, esterase, immunoglobulin and alkaline phosphatase activities and decreased alanine aminotransferase and aspartate aminotransferase activities compared with the control. Moreover, the dietary β-mannanase supplementation significantly improved intestinal morphology by increasing the muscle thickness and villi length. Salinity challenge showed that β-mannanase supplementation at 0.2 and 0.5 g/kg diet significantly increased fish survival compared with 0.8 and 0.0 g/kg diet. In conclusion, β-mannanase inclusion in S. rivulatus diets improved growth performance, feed utilization, non-specific immunity, intestinal morphology and fish quality. The recommended level of mannanase inclusion in S. rivulatus diets is 0.5 g/kg diet.